The present invention relates to a passive optical network system and a ranging method. More particularly, it relates to a passive optical network system in which communications are performed by wavelength division multiplexing and time division multiplexing in a passive optical network “PON” wherein a plurality of subscriber connection devices share an optical transmission line, and a ranging method in the passive optical network system.
A Passive Optical Network (PON) includes an Optical Line Termination (OLT), and a plurality of Optical Network Units (ONUs) or Optical Network Terminations (ONTs). Signals from terminals (PCs etc.) connected to the ONUs are sent to the OLT in such a way that the optical signals are optically multiplexed on an optical fiber leading to the OLT, through the ONUs, optical fibers and an optical splitter. After the OLT has executed various signal processes, communications from the terminals of certain ones of the ONUs with the terminals of the other ONUs of the pertinent PON or with the terminals of another network NW are performed. The above optical multiplexing covers the systems of Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), Code Division Multiplexing (CDM), and so forth. By way of example, G-PON stipulated by ITU-T Recommendation G. 984. 3 is a system wherein different wavelengths are used between in an uplink and in a downlink, and wherein the communications between an OLT (Optical Line Terminal) located at a central office and the ONUs (Optical Network Units) located at individual users are based on signals conforming to time division multiplexing (TDM) in which signal communication times are assigned to the respective ONUs.
In the TDM system, the individual ONUs are disposed at will within the range of, for example, optical fiber lengths of 0-20 km, 20 km-40 km, or 40 km-60 km as stipulated in Chapter 8 and Chapter 9 of ITU-T Recommendation G. 984. 1 (Non-patent Document 1 being ITU-T Recommendation G. 984. 1). Under this condition, therefore, transmission delays are different, and the optical signals might collide and interfere even when the individual ONUs have outputted the optical signals at the assigned signal communication times. For this reason, the optical signals from the individual ONUs are prevented from interfering on the optical fiber which leads to the OLT, in such a way that the delays of the output signals from the ONUs are adjusted as if these ONUs were disposed at equal distances (for example, 20 km), by employing a technique called “ranging” as stipulated in Chapter 10 of ITU-T Recommendation G. 984. 3 (Non-patent Document 3 being ITU-T Recommendation G. 984. 3). Further, the head of the signal from each ONU bears a guard time of 12 bytes for preventing the interference, a preamble which is utilized for the determination of the identification threshold value of a receiver and the extraction of a clock, and a delimiter which is utilized for identifying the delimitation of a received signal, as stipulated in Chapter 8. 8. 3 of Recommendation G. 984. 2 (Non-patent Document 2 being ITU-T Recommendation G. 984. 2).
By way of example, in the stipulation of Chapter 8. 2 of ITU-T Recommendation G. 984. 3, the signals which are transferred from the plurality of ONUs toward the OLT are called “uplink signals”, each of which contains the preamble, the delimiter and a payload signal. Besides, as shown in FIG. 8-2 in Chapter 8 of the recommendation, the guard time is set immediately before each uplink signal in order to prevent this uplink signal from colliding with a preceding burst signal. On the other hand, in accordance with the stipulation of Chapter 8. 1 of the recommendation, the signals which are transmitted from the OLT toward the plurality of ONUs are called “downlink signals”, each of which contains a frame synchronization pattern, a PLOAM field, a US Bandwidth MAP field and a frame payload. As indicated in Chapter 8. 1. 3. 6 of the recommendation, the OLT designates the uplink transmission permission timing of each ONU by using the field called “US Bandwidth MAP”. The US Bandwidth MAP field contains a “Start” value for designating the start of a transmission permission, and an “End” value for designating the end thereof, and the start and the end are respectively designated in byte units. The values are also called “grant values” in the sense that the transmission is permitted. In addition, the difference between the “End” value and the next “Start” value is a no-uplink signal field, which corresponds to the guard time. Incidentally, a plurality of bandwidth assignment units called “T-CONUs” can be allocated to the individual ONUs, and the designation of the uplink transmission permission timing is done every T-CONU.
In the ranging, the OLT requests the ONU to transmit a signal for distance measurement. When the ONU sends a distance measurement frame in reply, the OLT receives the signal of the frame, and it measures a time period since the transmission request for the distance measuring signal, till the reception of the signal of the distance measurement frame, that is, a reciprocating delay time, thereby to know how distant the ONU is from the OLT. Subsequently, in order that all the ONUs may be caused to seem at equal distances, the OLT instructs the individual ONUs to delay transmissions a time period called “equalization delay”. By way of example, in order to endow all the ONUs with a reciprocating delay time of 20 km, an equalization delay which is equal to [(the reciprocating delay time of 20 km)−(the measured reciprocating delay time)] is indicated to the ONUs as the instruction. The ONUs include circuits each of which transmits data with the fixed delay of the indicated equalization delay, and the uplink data transmissions are performed in compliance with the instruction so that all the ONUs may have the reciprocating delay time of 20 km.
Meanwhile, in the WDM system, a plurality of waves of different wavelengths are connected for both uplink signals and downlink signals between an OLT and ONUs, and the individual ONUs perform communications by receiving and transmitting specified wavelengths. The communications are performed by assigning the individual wavelengths from the OLT to the ONUs, whereby a communication bandwidth can be remarkably enhanced. One realization method for a WDM-PON in which the ONUs can be connected up to 32 units, is to assign one wavelength for each of the uplink and downlink of each ONU. That is, the number of wavelengths for use in one PON is set at double the maximum number (for example, 32 units) of the ONUs to be connected (32 wavelengths for the downlinks, and 32 wavelengths for the uplinks). In this case, one wavelength is occupied by one ONU, and hence, receivers which are equal in number to the wavelengths are required. On the other hand, however, the ranging procedure stated above is dispensed with, and the OLT need not include any burst signal receiver circuit requiring a high degree of technique as has been needed in the prior-art TDM-PON.